Stringiness and rheology of saliva substitutes

Saliva substitutes are man-made formulations commonly used in medicine, food, and pharmaceutical research to emulate the biochemical, tribological, and rheological properties of human saliva. Natural saliva possesses stringiness or spinnbarkeit, governed by extensional rheology response which cannot be evaluated or anticipated from the knowledge of shear rheology response. Even though extensional flows involving saliva are commonly encountered in situations such as swallowing, coughing, sneezing, licking, drooling, and blowing spit bubbles, rheological evaluations of saliva and its substitutes in most studies rely on shear viscosity alone. In this contribution, we provide a comprehensive examination of the shear and extensional rheology of twelve commercially available saliva substitutes and dry mouth treatments using rate-dependent torsional rheometry and dripping-onto-substrate (DoS) protocols, and evaluate their properties based on pioneering studies of saliva's viscoelasticity. Despite the majority of these formulations being marketed as having enhanced rheology, only three displayed measurable viscoelasticity and strain hardening, and did so at viscosities significantly higher than that of saliva.